Fix workaround in Segmented control by resetting SelectedIndex during template application (#699)

This commit modifies the `OnApplyTemplate` method in the `Segmented` class to set `SelectedIndex` to `-1` and then assign `_internalSelectedIndex` if `_hasLoaded` is false. This ensures the selected index is reset before applying the internal selection and marks the template as loaded.

Co-authored-by: Arlo <arlo.godfrey@outlook.com>

Author: AndrewKeepCoding

components/Segmented/src/Segmented/Segmented.cs

@@ -39,6 +39,7 @@ protected override void OnApplyTemplate()
         base.OnApplyTemplate();
         if (!_hasLoaded)
         {
+            SelectedIndex = -1;
             SelectedIndex = _internalSelectedIndex;
             _hasLoaded = true;
         }